Welcome to the forum. I will try to answer your questions as best I can.
1 – You can’t connect USB ports on multiple Aquacomputer (AC) devices to each other. Each one needs to go to its own USB port to the mobo. The solution is to install an internal USB 2.0 hub that is Sata or Molex powered. Connect the hub to one of your mobo USB2.0 ports then connect the AC device’s USB ports to the hub. AC sells the
HUBBY7 which works very well for this purpose. The NZXT AC-IUSBH-M3 USB2.0 Hub is a tried and true alternative, which, unlike the Hubby7, is in stock at multiple online sites. I just bought one
HERE because I could not find a HUbby7 in stock in the US. Keep away from cheap USB2.0 hubs on Amazon that aren’t Sata or Molex powered.
2 – With your devices, yes you can ignore aquabus. Aquabus is a legacy serial control protocol that has limited bandwidth. It is mainly used to connect devices to an Aquaero. The “Next” series of devices rely on USB communication with the Aquasuite (AQS) software. Some devices, like the Octo, still have an aquabus port but this is for connecting the Octo to an Aquaero, which will make the Octo a slave of the Aquaero controller.
The aquabus port on the D5 Next is used to connect the pump to an Aquaero. The manual says it can also be used as an RGBpx port (not at the same time). I am not clear on how this works because AC reps have said many times that aquabus has limited bandwidth and cannot carry the amount of data required by RGBpx. Maybe someone else can chime in here and clarify this. The aquabus port on the High Flow Next is for connection to an Aquaero. It has no other function.
3. Since you have a High Flow Next, which will be connected to the mobo via the USB2.0 hub, there is no need to use the flow sensor headers on the Octo or the D5 Next. There actually is a reason to connect the High Flow next Signal output to the D5 Next Fan/Flow header (with cable #53294) but it’s kind of complicated to explain. It has to do with the D5 Next virtual flow sensor being used as the data source for the pump’s Alarm function. If you want to know the details of this let me know and I will try to explain it.
4 - I don’t own an Aquaero so I don’t know a lot about it. It’s an older controller and I don’t think they are available anymore. You can still find them in stock here and there. AC said they are not making them now because some of the component prices have skyrocketed and some have 18-month lead times. They may resume manufacturing at some point in the future, or maybe they will release a new model.
The Aquaero does do some things that the Octo can’t, starting with being able to drive DC and PWM fans. The
CaliTemp sensors can only be used with an Aquaero. The Aquaero also has a relay output and an Aquastream Bridge connector for the
Aquastream pumps. The Aquaero is a complex device. If you are operating Linux it may be a consideration because the Next devices with USB require the Aquasuite service to be running which requires Windows. If you are running Windows, it is doubtful that an Aquaero would work better unless you have some unusual requirements.
5 - Aquasuite cannot control your mobo LEDs. It can control DRGB LEDs in fans, LED strips, etc. They have adapters to go to and from their RGBpx connectors (Molex Picoblade) to the standard (and horrible) DRGB connectors. Each RGBpx port can drive 90 DRGB LEDs.
6 – Yes. The Leakshield sensors will report the High Flow Next flow rate and the D5 Next Pump speed. All the devices have assignable virtual sensors as well.
Note - be sure to check out the Playground area of Aquasuite. This is where the real power lies. You can combine and process sensor data here and build virtual sensors that are based on real sensors. A very basic example is subtracting ambient temp from coolant temp, then using the difference between these temps (delta-t) as the control source for your fans. You could have the fan PWM ports track delta-t but also kick up to a higher speed if your CPU and/or GPU exceed a threshold temp for a period of time. If you have a temp sensor at the inlet and outlet of a rad, and you know the flow rate, you can calculate how many watts of heat the rad is dissipating. The possibilities are endless.